In color display devices employed for image display on computers or TV sets, a plasma display device having a plasma display panel (hereinafter referred to as a PDP) has recently received considerable attention as a color display device with large sized screen but lightweight body due to its low-profile structure.
A plasma display device displays images in full color by performing an additive color mixing process on red, green, and blue—known as the three primary colors. To realize the full color display, the plasma display device has phosphor layers that are respectively prepared for emitting red (R), green (G), and blue (B) of the three primary colors. A phosphor layer is formed of phosphor particles. The phosphor particles are excited by ultraviolet light generated in a discharge cell in the PDP, so that visible light of red, green, and blue are produced.
A well-known combination of chemical compounds for the phosphors above are: (Y,Gd) BO3:Eu3+ and Y2O3:Eu3+ for emitting red light and positively charged; Zn2SiO4:Mn2+ for emitting green light and negatively charged; and BaMgAl10O17:Eu2+ for emitting blue light and positively charged (for example, see O plus E; Feb, 1996, No. 195 pp 99 and 100.)
It is also disclosed that a phosphor Zn2SiO4:Mn2+, for example, is produced through solid phase reaction. In this method, specified materials are mixed, and baked at high temperatures of at least 1,000° C. (for example, see Phosphor Handbook, pp 219 and 220, Ohmsha). After the baked phosphor particles are lightly milled so that aggregated particles loosen and breakage of crystals causing luminance degradation does not occur, they are classified so that red and green phosphors have an average particle diameter of 2-5 μm, and the blue phosphor 3-10 μm.
The phosphor particles are milled and classified for the following reason. In general, when phosphor layers are formed on a PDP, a method of screen-printing a paste of phosphor particles of each color, or an ink jet method of discharging a phosphor paste from a nozzle for application is used. In application of the paste, the smaller and more uniform diameters of phosphor particles (i.e. a uniform particle size distribution) can easily provide a smoother coated surface. Particularly in forming phosphor layers using an ink jet method, non-uniform application or clogging of a nozzle may occur because large aggregates are included in the phosphors if the particles are not lightly milled and classified (for example, see Japanese Patent Unexamined Publication No. H06-273425). That is, when phosphor particles have smaller and more uniform diameters and shapes approximating to a sphere, the coated surface is smoother. The smoother coated surface increases the packing density of the phosphor particles in a phosphor layer. As a result, the emission surface area of the particles increases, and the luminance of the plasma display device increases.
Additionally disclosed is another PDP in which a Zn2SiO4:Mn green phosphor negatively charged on its surface and a positively charged green phosphor ReBO3:Tb (where Re are rare-earth elements, e.g. Sc, Y, La, Ce, and Gd) are mixed to produce an apparently positively charged phosphor, and a BaMgAl10O17:Eu blue phosphor and a (Y,Gd) BO3:Eu red phosphor that are positively charged are used (for example, see Japanese Patent Unexamined Publication No. 2001-236893).
However, a plasma display panel produced by using BaMgAl10O17:Eu as a blue phosphor, Zn2SiO4:Mn as a green phosphor, and (Y,Gd) BO3:Eu or Y2O3:Eu as a red phosphor has the following problem.
In these phosphors, blue and red phosphors are positively charged on their surfaces. However, the green phosphor made of Zn2SiO4:Mn is negatively charged on its surface. This is because the ratio of SiO2 to ZnO (1.5ZnO/SiO2) is larger than the stoichiometric mixture ratio (2ZnO/SiO2), and thus the surface of the Zn2SiO4:Mn crystal is covered with SiO2. When negatively charged phosphor and positively charged phosphor coexist in a PDP, negative charges remain only on the negatively charged phosphor at driving the panel especially after the entire panel is light up and put off. For this reason, in application of voltages for display, variation in discharge, or addressing failure, i.e. no discharge, occurs.
Additionally, because the surface of Zn2SiO4:Mn is covered with SiO2, gas can easily be adsorbed. For this reason, Zn2SiO4:Mn adsorbs a large amount of water (H2O), carbon monoxide (CO), carbon dioxide (CO2), or hydrocarbon (CxHy). Theses gases are emitted into the panel in an aging process after a panel sealing process. These gases are adsorbed onto the surface of MgO, causing addressing failure and deteriorating the discharge characteristics. Additionally, these gases adsorbed onto the surface of the blue phosphor BaMgAl10O17:Eu, causing luminance degradation of the blue phosphor and color shift, i.e. decrease in the color temperature of the panel caused by increase in the y value of chromaticity. Further, Ne positive ions and CH positive ions collide with the negatively charged green phosphor, thus causing luminance degradation of the phosphor.
On the other hand, even when a green phosphor Zn2SiO4:Mn negatively charged on its surface and a green phosphor ReBO3:Tb (where Re are rare-earth elements, e.g. Sc, Y, La, Ce, and Gd) positively charged are mixed to produce an apparently positively charged phosphor, and a positively charged blue phosphor BaMgAl10O17:Eu and a positively charged red phosphor (Y,Gd) BO3:Eu are used, the following problem occurs. These phosphors contain Zn2SiO4 likely to adsorb water (H2O) and hydrocarbon (CxHy), and BaMgAl10O17 likely to adsorb water (H2O). For this reason, water (H2O), carbon monoxide (CO), carbon dioxide (CO2) and hydrocarbon (CxHy) emitted into the panel during discharge deteriorate MgO and cause addressing failure and degrade discharge characteristics, further causing the luminance degradation and color shift of BaMgAl10O17:Eu.
The present invention addresses these problems, and aims to provide a plasma display device in which phosphor layers are stably formed and has no degradation of luminance and chromaticity, and stable discharge characteristics.